1. Field of the Invention
The present invention relates to an integrator, and more particularly, to a multi-channel integrator.
2. Description of Related Art
With the blooming development in the electronic technology, and the prevalence of wireless communication and the internet, a variety of electronic devices are becoming indispensable in people's day-to-day life and work. However, it is rather difficult to operate the most common input-output (I/O) interface, such as a keyboard or a mouse. Compared with a keyboard and a mouse, a touch panel is a simpler I/O interface. Therefore, the touch panel is usually applied as a man-machine interface between a man and an electronic device so as to perform some control operation.
Generally speaking, the touch panel can be classified into a resistive touch panel, an optics touch panel, and a capacitive touch panel, etc. On the other hand, the touch panel can also be classified into a current-type touch panel and a charge type touch panel when being classified in a readout manner. FIG. 1 is a schematic diagram of a capacitive touch panel and a traditional readout circuit. A common capacitive touch panel 110 has a plurality of sensor lines both in the Y-axis direction and in the X-axis direction. A coupling capacitor Cp is formed between one of the sensor lines in the Y-axis direction and one of the sensor lines in the X-axis direction.
Each of the sensor lines is with an integrator 120. Besides, an operational amplifier (OP-AMP) 122 and a feedback capacitor Cfb are disposed in each of the integrators 120. In the beginning, a non-inverting input terminal of each of the OP-AMPs 122 receives a 0V reference voltage Vref, and each of switches 123 is turned on. Thus, each of the sensor lines is charged to 0V. Next, each of the integrators 120 turns off the switch 123 thereof so as to perform a readout operation. If no conductor, such as a finger, is approached to or touches the touch panel 110 during a turn off period of the switch 123, the voltage of two terminals of the coupling capacitor Cp are changed to 5V by the integrators 120 in the Y-axis direction and in the X-axis direction as the reference voltage Vref is transferred from 0V to 5V. Since there is no need to charge and discharge the coupling capacitor Cp, the variation that the reference voltage Vref is transferred from 0V to 5V is reflected on the output of integrator 120. After each of the integrators 120 complete the readout operation, each of the switches 123 are turned on again. And the above-mentioned steps are repeated all over again.
When a conductor, such as a finger, touches the touch panel 110, an extra capacitor Cf is formed at a corresponding location as shown in FIG. 1. During the turn off period of the switch 123, when the reference voltage Vref is transferred from 0V to 5V, the corresponding integrator 120 needs to charge and discharge the extra capacitor Cf through one of the sensor lines. Hence, the output OUT of the integrator 120 corresponding to the extra capacitor Cf changes as the reference voltage Vref is transferred from 0V to 5V, which is represented as OUT=5+[(5V−0V)×Cf]/Cfb. The integrator 120 transmits the readout result to following circuits, which includes a digital to analog convertor and an image processing circuit (not shown), so that the location coordinate is determined. Hence, the touch location is determined upon the difference readout data between the sensor line with the extra capacitor Cf and the sensor line without the extra capacitor Cf.
From the foregoing equation, the larger the extra capacitor Cf is, the larger the feedback capacitor Cfb is needed. Otherwise, the touch location can not be determined due to the output saturation of the integrator 120. However, in order to prevent the output saturation of the integrator 120, the capacitance of the feedback capacitor Cfb is needed to be increased, i.e. the area of the feedback capacitor Cfb is needed to be increased. Since each of the sensor lines needs one integrator 120, the chip area occupied by the integrator 120 is significantly large.